1. Field of the Invention
This invention is related with an image reproduction speed conversion apparatus. More specifically, this invention is related with the image reproduction speed conversion apparatus, which can change the vertical scanning cycle and the horizontal scanning cycle of image signals, of LD (Laser Vision Disk), VTR (Video Tape Recorder) an so on, and which can perform a variable speed reproduction.
2. Description of the Related Art
A frame-synchronizer is one example of an image reproduction speed conversion apparatus which converts the horizontal scanning cycle of an image signal. A frame-synchronizer will be explained hereinbelow.
A frame-synchronizer is the apparatus which makes synchronizations of two video signals match to each other. The synchronous signal cycle of each video signal generally differs within the range of the standard.
Therefore, the synchronous signal cycle of one video signal is made to be a standard (standard synchronous signal cycle). The video signal of another side is A/D-converted (Analogue to Digital converted) by the synchronous signal cycle of the concerned video signal. The A/D-converted video signal is written in a frame memory, and is read out by the signal which is synchronized with the standard synchronous signal cycle. In this way, the video signals are synchronized to each other.
Generally, the synchronous signal cycles of two video signals differ with each other minimally. For this reason, the write-in timing and the read-out timing to a frame memory approach to each other by very small increments. Even so, a write-in timing may pass a read-out timing or a read-out timing may pass a write-in timing. Thus a surpassing state arises.
For example, data will be read before updating data of a frame memory when a write-in speed (write-in timing) is lower than a read-out speed (read-out timing) as shown in FIG. 1A. Thus, previous data will be erroneously read. Therefore, a picture corresponding to the present frame is displayed on the upper portion of the screen, while the picture corresponding to the previous frame is displayed on the bottom thereof. More concretely, the picture of the third frame is displayed in the first half of the time boundary by the arrow 51 in FIG. 1A. The picture of the second frame which should be displayed for the last time is displayed in the second half thereof.
Data corresponding to the picture of the next frame is written in the middle of data being read from the frame memory, when the write-in speed is higher than the read-out speed, as shown in FIG. 1B. Thus, when this data is read out and displayed on a display device etc., a gap in the picture arises, and thus a gap of the color phase arises. More concretely, the picture of the third frame is displayed in the first half of the boundary of time shown by the arrow 52 in FIG. 1B, while the picture of the fourth frame which should be displayed for the next time is displayed in the second half thereof.
Surpassing-processing to be performed when such a surpassing state arises is as follows.
a) A write-in processing is not performed to the frame with respect to which it is predicted that a surpassing state arises. or PA1 b) An operation of writing data corresponding to the scanning line with respect to which it is predicted that a surpassing state arises, is not performed.
Gaps of the color phase arise in the surpassing-processing of above-mentioned a) and b). In order to maintain the continuity of the color phase, an exclusive color phase processing is necessarily performed.
Moreover, in the applied apparatus using a field memory instead of a frame memory, passing of the field arises from the moment of passing. Thus, the field inversion phenomenon arises. Namely, the phenomena that the picture of the second field is displayed on the first field, and the picture of the first field is displayed on the second field, arise. For this reason, for example, the surpassing-processing is performed by shifting the first field display position by one scanning line, to prevent the inversion of the field.
By the way, with respect to the television signal of NTSC (National Television System Committee) system, for example in usual TV (Television) display device, 262.5 scanning lines per field are required. The number of effective scanning lines which appears on an actual screen is 245. The remainder is spent in a vertical-retrace term and are therefore invalid. For this reason, one field consists of 245 scanning lines in the actual picture plane (real display screen).
However, the surpassing-processing is performed whenever a surpassing state arises in the above-mentioned image reproduction speed conversion apparatus. That is to say, a surpassing-processing will be always performed even when a surpassing state arises in an invalid screen portion e.g., in other than the effective screen portion (which is actually displayed). Thus, there is a problem that picture degradation of the effective screen is all the more likely when performing a surpassing-processing.